Spin State Behavior of A Spin-Crossover Iron(II) Complex with N,N (cid:48) -Disubstituted 2,6-bis(pyrazol-3-yl)pyridine: A Combined Study by X-ray Di ﬀ raction and NMR Spectroscopy

: A series of three di ﬀ erent solvatomorphs of a new iron(II) complex with N,N (cid:48) -disubstituted 2,6-bis(pyrazol-3-yl)pyridine, including those with the same lattice solvent, has been identiﬁed by X-ray di ﬀ raction under the same crystallization conditions with the metal ion trapped in the di ﬀ erent spin states. A thermally induced switching between them, however, occurs in a solution, as unambiguously conﬁrmed by the Evans technique and an analysis of paramagnetic chemical shifts, both based on variable-temperature NMR spectroscopy. The observed stabilization of the high-spin state by an electron-donating substituent contributes to the controversial results for the iron(II) complexes of 2,6-bis(pyrazol-3-yl)pyridines, preventing ‘molecular’ design of their spin-crossover activity; the synthesized complex being only the fourth of the spin-crossover (SCO)-active kind with an N,N (cid:48) -disubstituted ligand.

Until very recently [18], however, any substituent in this position of 3-bpp resulted only in HS complexes of iron(II) mostly [22] through its steric [23] bulk, as generally accepted in the SCO [21]. Askew from the common wisdom, the ligand to obtain the first SCO-active complexes of N,N′disubstituted 3-bpp [18] was created by size-tailoring ortho-substituents in 2,6-functionalized Nphenyl groups. Encouraged by the success, here we report an iron(II) complex of the same ligand design [18] L but with a different substituent, an electron-donating bulky t-butyl group, in the fifth position of the pyrazol-3-yl moiety (Scheme 1) to probe its effect on the spin state of the iron(II) ion by two complementary [15] techniques [11,24], X-ray diffraction and NMR spectroscopy. X-ray diffraction is an indispensable tool in search for structure-function relationships [21] in SCO-active complexes [3,17,25], including those of 3-bpp [15], with operative crystal packing forces that may induce a desired abrupt SCO with a thermal hysteresis [9,10] or, instead, 'block' it from occurring [15,26]. The latter makes screening of solutions [21] by NMR spectroscopy, which is behind the popular Evans method [27] or a more exotic analysis [28][29][30][31][32] of paramagnetic chemical shifts [33], crucial for a successful 'truly molecular' design [21] of SCO compounds. Here, both of these techniques are applied for studying the spin-state behavior of the new iron(II) complex of N,N′disubstituted 3-bpp, [Fe(L)2](BF4)2, to get deeper insights into its potential thermally induced SCO [18] ability as a function of chemical modifications of the ligand similar to those gathered for 1-bpp [16]. Scheme 1. Ligand L, 2,6-bis(5-tert-butyl-1-(2,6-dichlorophenyl)-1H-pyrazol-3-yl)pyridine.

Synthesis
All procedures were carried out in air and in solvents purchased from commercial sources and purified by distilling from conventional drying agents under an argon atmosphere prior to use. Diethyl 2,6-pyridinedicarboxylate was obtained commercially (Sigma-Aldrich), and 2,6dichlorophenylhydrazine was synthesized from commercially available 2,6-dichloroanilines using a standard diazotization protocol with a subsequent reduction by SnCl2 [18]. High-resolution massspectra were recorded on a Bruker Daltonics microTOF electrospray ionization-time-of-flight (ESI-TOF) mass spectrometer (Bremen, Germany).

Synthesis of the ligand L
In the first step, 1,1′-(pyridine-2,6-diyl)bis(4,4-dimethylpentane-1,3-dione) was obtained as described earlier [34] (Scheme 2). To do so, pinacolone (4.36 mL, 35 mmol) was added to a suspension of NaH as a 60% suspension in oil (1.4 g, 35 mmol) and of diethyl 2,6-pyridinedicarboxylate (2.23 g, 10 mmol) in dry THF (100 mL), which was then heated to reflux. After 3 h, the solvent was removed under vacuum, the resulting sodium salt was suspended in water, and the suspension was treated with HCl until the pH value reached 4. The following extraction with CHCl3, drying of the solution with Na2SO4, and removal of the solvent in vacuo yielded the target tetraketone as a bright yellow solid, which was of a sufficient purity for further transformations. Yield: 2.51 g (76%). Anal. Calc. for

Synthesis
All procedures were carried out in air and in solvents purchased from commercial sources and purified by distilling from conventional drying agents under an argon atmosphere prior to use. Diethyl 2,6-pyridinedicarboxylate was obtained commercially (Sigma-Aldrich), and 2,6-dichlorophenylhydrazine was synthesized from commercially available 2,6-dichloroanilines using a standard diazotization protocol with a subsequent reduction by SnCl 2 [18]. High-resolution mass-spectra were recorded on a Bruker Daltonics microTOF electrospray ionization-time-of-flight (ESI-TOF) mass spectrometer (Bremen, germany).

X-ray Crystallography
Single crystals of [Fe(L)2](BF4)2·2CH3CN (red), [Fe(L)2](BF4)2·0.5H2O (red) and [Fe(L)2](BF4)2·2CH3CN (yellow) were grown by slow diffusion of diethyl ether vapor into an acetonitrile solution of this complex kept on air for two months. X-ray diffraction data were collected at 120 K with a Bruker APEX2 DUO CCD diffractometer (Karlsruhe, Germany), using the graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). For [Fe(L)2](BF4)2·2CH3CN (yellow), they were also collected at 293 K. An attempt to do so for [Fe(L)2](BF4)2·2CH3CN (red) and [Fe(L)]2(BF4)2·0.5H2O (red) was unsuccessful, as heating their single crystals to higher temperatures resulted in their deterioration. Using Olex2 [35], the structures were solved with the ShelXT structure solution program [36] using Intrinsic Phasing and refined with XL refinement package [37] using Least Squares minimization. Hydrogen atoms of water molecules in [Fe(L)2](BF4)2·0.5H2O (red) were located in difference Fourier synthesis. Positions of other hydrogen atoms were calculated, and they all were refined in the isotropic approximation in the riding model. Crystal data and structure refinement parameters for the three crystallosolvates are given in Table 1 The ligand L (Scheme 2) was synthesized from the obtained 1,1 -(pyridine-2,6-diyl)bis(4,4dimethylpentane-1,3-dione) (0.331 g, 1 mmol), which was dissolved in 10 mL of acetic acid together with 2,6-dichlorophenylhydrazine (0.407 g, 2.3 mmol) to produce a yellow solution. After its stirring at room temperature overnight, a white precipitate appeared. This precipitate was filtered, washed with acetic acid and water, and dried under vacuum. The product was used without further purification. Yield: 478 mg (78% In a 20 mL vial, iron tetrafluoroborate hexahydrate (0.0337 g, 0.1 mmol) and a ligand L (0.122 g, 0.2 mmol) were mixed in 3 mL of acetonitrile and stirred for 1 h. Volatiles were removed under vacuum. For further purification, the solid residue was dissolved in acetonitrile, and diethyl ether was added portion-wise until a precipitate appeared. The mixture was then stored for 12 h at

X-ray Crystallography
Single crystals of [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red), [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red) and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) were grown by slow diffusion of diethyl ether vapor into an acetonitrile solution of this complex kept on air for two months. X-ray diffraction data were collected at 120 K with a Bruker APEX2 DUO CCD diffractometer (Karlsruhe, germany), using the graphite monochromated Mo-Kα radiation (l = 0.71073 Å). For [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow), they were also collected at 293 K. An attempt to do so for [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red) and [Fe(L)] 2 (BF 4 ) 2 ·0.5H 2 O (red) was unsuccessful, as heating their single crystals to higher temperatures resulted in their deterioration. Using Olex2 [35], the structures were solved with the ShelXT structure solution program [36] using Intrinsic Phasing and refined with XL refinement package [37] using Least Squares minimization. Hydrogen atoms of water molecules in [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red) were located in difference Fourier synthesis. Positions of other hydrogen atoms were calculated, and they all were refined in the isotropic approximation in the riding model. Crystal data and structure refinement parameters for the three crystallosolvates are given in Table 1

Evans Method
Variable-temperature magnetic susceptibility in acetonitrile-d 3 and dichloromethane-d 2 was probed by the Evans method [27,38] in the accessible temperature ranges of 235-345 K and 190-300 K, respectively. In all cases, a Wilmad NMR tube equipped with a coaxial insert was used. The inner (reference) tube was filled with the appropriate solvent with approximately 1% of Me 4 Si, and the outer tube contained the solution of [Fe(L) 2 ](BF 4 ) 2 (≈1-5 mg/cm 3 ) in the same solvent with the same concentration of Me 4 Si. Molar magnetic susceptibility was calculated from the difference between the chemical shift of Me 4 Si in the pure solvent and its shift in a solution of the complex (∆δ in Hz) in this solvent using the following equation: (M-molar weight of the iron(II) complex, g/mol; ν 0 -frequency of the spectrometer, Hz; S f -shape factor of the magnet (4π/3); c-concentration of the complex, g/cm 3 ; χ M dia -molar diamagnetic contribution to the paramagnetic susceptibility calculated using the Pascal's constant [39]). The concentration c was recalculated for each temperature point accounting for the change in the density of the solvent ρ: c T = m s ρ/m sol , where m s is the mass of the complex and m sol is the mass of the solution.

Temperature-Dependence of Chemical Shifts
Chemical shifts in 1 H NMR spectra from the solutions of [Fe(L) 2 ](BF 4 ) 2 in acetonitrile-d 3 and dichloromethane-d 2 were analyzed in the accessible temperature ranges of 235-345 K and 190-300 K, respectively. For a compound adopting two different spin states, the observed chemical shift of a given nucleus in the 1 H NMR spectrum is a weighted average of those for LS and HS species (η LS and η HS are their populations) given that spin state switching is a fast process in the NMR timescale (which is almost [29] always the case [28]): For the iron(II) complexes with the diamagnetic LS state, this chemical shift (in ppm) can be expressed as: A good approximation for the diamagnetic contribution δ dia , which is the same for the LS and HS states, is a chemical shift of the same nucleus in the NMR spectrum of a free ligand [40]. Therefore, paramagnetic contribution δ HS par was measured as a difference between the chemical shifts observed for [Fe(L) 2 ](BF 4 ) 2 and those for the ligand L. At high temperatures (T > 200 K), the paramagnetic chemical shifts for a paramagnetic complex with low magnetic anisotropy [33,41] in a pure spin state follow the linear dependence on the inverse temperature (the Curie law): δ HS par = A + BT −1 .

Analysis of Theoretical Chemical Shifts
Theoretical chemical shifts in the 1 H NMR spectra for [Fe(L) 2 ](BF 4 ) 2 in acetonitrile-d 3 were obtained as follows: δ obs = δ dia + δ CS + δ PCS . Isotropic paramagnetic (contact) contribution δ CS , which arises from spin polarization conveyed through molecular orbitals, was evaluated with the following equation: (S-electron spin; g N -nuclear g-factor; µ B -Borh magneton; µ N -nuclear magneton). The g-tensor and isotropic values of hyperfine interaction tensors A iso were taken from quantum chemical calculations [42], using optimized geometry (as described below) of the complex in the HS state. The pseudocontact contribution δ PCS , which arises from dipolar coupling between magnetic moments of a nucleus and of an unpaired electron [43][44][45], was estimated by fitting the observed chemical shifts to the following equation: (θ i and r i -polar coordinates of the nuclei in the coordinate frame of the magnetic susceptibility tensor χ; ∆χ ax -axial anisotropy of χ).

Quantum Chemistry
Quantum chemical calculations were performed with the ORCA package, v. 4.2 [46]. Using X-ray diffraction geometries from [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red) and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) as a starting point for the following geometry optimization, the geometry of the species [Fe(L) 2 ] 2+ was optimized with a TPSSh functional [47,48], which provides good results for the energy difference between the two spin states in (pseudo)octahedral iron(II) complexes [49], and a basis set def2-TZVP [50]. To speed up the calculations, the RIJCOSX approximation [51] with a def2/J fitting basis [52] set was used. Extra tight thresholds for forces and displacements were applied. The solvation effects were included using the Conductor-like Polarizable Continuum Model, as implemented in the ORCA package, v. 4.2 [46], with acetonitrile and water as solvents. The resulting geometry of [Fe(L) 2 ] 2+ in the HS state was used to compute g-tensor and isotropic values of hyperfine interaction tensors A iso [42] with the hybrid PBE0 functional (providing good results for hydrogen spin densities [42] used in calculating chemical shifts in the NMR spectra of paramagnetic compounds) [28] and the basis set def2-TZVP [50].

Results and Discussion
For the synthesis of the ligand L, a one-step cyclization [34] of 1,1 -(pyridine-2,6-diyl)bis(4,4dimethylpentane-1,3-dione) and 2,6-dichloro-phenylhydrazine was carried out in glacial acetic acid. Although it could lead to two possible regioisomers with a different location of aryl substituents, only the target isomer, 2,6-bis(5-tert-butyl-1-(2,6-dichlorophenyl)-1H-pyrazol-3-yl)pyridine (L), was obtained in a high yield. Its subsequent reaction with Fe[BF 4 ] 2 ·6H 2 O in acetonitrile at room temperature readily produced the iron(II) complex [Fe(L) 2 ](BF 4 ) 2 , as confirmed by elemental analysis and NMR spectroscopy. To characterize it by X-ray diffraction, attempts have been made to grow high-quality single crystals by slow diffusion of diethyl ether vapor into a solution of the complex in acetonitrile. Those resulted in a mixture of red and yellow crystals, the colors of which are indicative [3] of the LS and HS states of the metal(II) ion, respectively, rarely [18] found together in iron(II) complexes with N,N -disubstituted 3-bpp ligands [21,22,26,[53][54][55]. The X-ray diffraction data collected for them at 120 K ( Figure 1 . The former two with the same color and habitus cannot be distinguished by a naked eye, thus precluding their isolation in quantities needed for other solid-state techniques used in SCO research [24,56], such as variable-temperature magnetochemistry or UV-vis spectroscopy. One of the key features known to play an important role in the solid-state behavior of metal complexes with a potential SCO activity [57] is a 'terpyridine embrace' packing motif [58] arising from face-to-face stacking interactions between neighboring complex species [58,59]. In our case, there are no such motifs in all the three solvatomorphs, although 'terpyridine-embrace'-like patterns can be envisaged in [Fe(L)2](BF4)2·0.5H2O (red) and [Fe(L) 2](BF4)2·2CH3CN (yellow) (Figure 2). An overall crystal structure is, however, better described as infinite spirals formed by the cations  Table 1) with much smaller solvent cavities [60] occupied by small water molecules [60] and, apparently, more efficient intermolecular interactions favoring the LS state in One of the key features known to play an important role in the solid-state behavior of metal complexes with a potential SCO activity [57] is a 'terpyridine embrace' packing motif [58] arising from face-to-face stacking interactions between neighboring complex species [58,59]. In our case, there are no such motifs in all the three solvatomorphs, although 'terpyridine-embrace'-like patterns can be envisaged in [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red) and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) (Figure 2). An overall crystal structure is, however, better described as infinite spirals formed by the cations [Fe(L) 2 ] 2+ along the crystallographic axis a in [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red) and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) and the axis c in [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red) with the shortest distance between the iron(II) ions equal to 12.900(4), 12.5743(4), and 12.2443(6) Å, respectively. The tetrafluoroborate counterions and appropriate solvent molecules, which are disordered in one of the acetonitrile solvatomorphs, occur between these spirals (Figure 2) to produce a more dense crystal packing in [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red) ( Table 1) with much smaller solvent cavities [60] occupied by small water molecules [60] and, apparently, more efficient intermolecular interactions favoring the LS state in similar iron(II) complexes [61,62]. The resulting differences in the crystal environment of the cation [Fe(L)2] 2+ in the three solvatomorphs are nicely visualized by Hirshfeld surfaces [63,64], which divide the crystal into 'molecular' domains with a dominating contribution to the electron density coming from a particular species, and their 2D fingerprint plots [65], mapping different types of intermolecular interactions and their occurrence by the distances from a point on the Hirshfeld surface to the closest atom inside (di) and outside (de) it. Adjusted for the sum of van-der-Waals radii of such pairs of atoms (dnorm), these distances allow for color coding the strength of corresponding interactions on the Hirshfeld surface ( Figure 3) (BF4)2·2CH3CN (yellow)), is clearly behind the different spin state adopted by the metal ion [60], as follows from the colors of these solvatomorphs that are characteristic of LS and HS iron(II) complexes [3], respectively, as well as from X-ray diffraction data collected for them at 120 K (Table 2) [66]. On the Hirshfeld surfaces (a,c), intermolecular interactions with interatomic distances below, equal or above a sum of van-der-Waals radii are shown by red, white, and blue areas, respectively. green and blue areas on fingerprint plots (b,d) stand for higher and lower concentration of points corresponding to (di, de) pairs. For the third solvatomorph, see Figure S5. The different crystal environment of the cation [Fe(L) 2 ] 2+ in these solvatomorphs, including those with the same composition ([Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red) and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow)), is clearly behind the different spin state adopted by the metal ion [60], as follows from the colors of these solvatomorphs that are characteristic of LS and HS iron(II) complexes [3], respectively, as well as from X-ray diffraction data collected for them at 120 K (Table 2). Indeed, the Fe-N bond lengths in [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red), [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red), and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) fall within the ranges typical for, respectively, LS and HS complexes of iron(II) with heterocyclic N-donor ligands [15] that form an N6 coordination environment responding to the spin state of the metal ion [67]. While in the LS state such complexes tend to be octahedral, their HS state is prone to a distortion towards a trigonal prism [68]. For the complexes [Fe(3-bpp) 2 ] 2+ , it is usually described by the 'twist' angle (θ) between the planes of the two 3-bpp ligands and the 'rotation' angle N Py -Fe-N Py (φ) [69]. In any of the three solvatomorphs, these angles are close to 90 • and 180 • featured by an ideal octahedron ( Table 2)   A more elegant way to quantify this distortion [67,69], which is often [70] a good indicator of the spin state [26], is to use continuous symmetry measures [67]. They measure how close is the shape of the coordination polyhedron to a reference shape, such as an ideal octahedron (OC-6). The lower the value of an appropriate symmetry measure, the better the fit to a chosen polyhedron is. For example, S(OC-6) would be zero for an ideal octahedron. While an SCO in iron(II) complexes with an N6 donor set is usually associated with a trigonal twist distortion [68], an edge-bicapped tetrahedron (ebcT-6) [69] better matches the coordination environment of the metal ion in HS complexes with two meridian tridentate ligands [69]. For [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red), [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red), and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow), the symmetry measures S(OC-6) and S(ebcT-6) evaluated from the X-ray diffraction data at 120 K (and typical for iron(II) complexes of 3-bpp [26]) clearly distinguish the HS solvatomorph from the two others [26] (Table 2, Figure 4) by the coordination geometry that is more distorted towards an edge-bicapped tetrahedron [69] but hardly enough to prevent an SCO [13,18].  and S(ebcT-6) are octahedral and edge-bicapped tetrahedral symmetry measures, respectively, with a line representing minimum distortion pathway between these reference shapes. Open circles and black squares correspond to the coordination geometry from X-ray diffraction ( Table 2) and quantum chemistry calculations (Table S1) (Table 2). It, however, led to an anisotropic thermal expansion of the unit cell by 3.5%, which is slightly larger than expected for SCO compounds [72]. The lattice parameter a running along the infinite spirals of the cations [Fe(L)2] 2+ ( Figure S7) experiences a much larger increase ( Figure S8) than the parameters b and c (18% vs. 8-9%), thus mirroring the changes in the shortest distance between the iron(II) ions in these spirals (from 12.5743(4) to 12.764(3) Å) and between them (from 13.6079(4) to 13.727(3) Å and from 14.6224(6) to 14.742(6) Å in two different directions; Figure S8).
A possibility for it to undergo an SCO decoupled from crystal packing [15,26] or substrate [73,74] effects, such as in solutions [21,33], was supported by quantum chemical calculations of the species [Fe(L)2] 2+ in solvents found in the three solvatomorphs (acetonitrile and water), as accounted by the CPCM model [46,75] Table 2 and Table S1); the largest difference in the Fe-N bond lengths of 0.085 −0.154 Å is observed for the bonds Fe-NPz in the HS species of the complex. The distortion of the coordination polyhedron towards an edge-bicapped tetrahedron, as quantified by both the 'twist' and 'rotation' angles and the symmetry measures S(Oh) and S(ebcT) (Table S1, Figure 4), is very similar to that from the X-ray diffraction data collected for the solvatomorphs at 120 K. The and S(ebcT-6) are octahedral and edge-bicapped tetrahedral symmetry measures, respectively, with a line representing minimum distortion pathway between these reference shapes. Open circles and black squares correspond to the coordination geometry from X-ray diffraction ( Table 2) and quantum chemistry calculations (Table S1), respectively.
Although an SCO-activity in similar iron(II) complexes of 3-bpp with N-aryl substituents [18,22] was previously attributed to the rotation of these substituents from the pyrazol-3-yl plane, an average rotation angle (γ) remains roughly the same in all the three solvatomorphs of [Fe(L) 2 ](BF 4 ) 2 ( Table 2) (Table 2). It, however, led to an anisotropic thermal expansion of the unit cell by 3.5%, which is slightly larger than expected for SCO compounds [72]. The lattice parameter a running along the infinite spirals of the cations [Fe(L) 2 ] 2+ ( Figure S7) experiences a much larger increase ( Figure S8) than the parameters b and c (18% vs. 8-9%), thus mirroring the changes in the shortest distance between the iron(II) ions in these spirals (from 12.5743(4) to 12.764(3) Å) and between them (from 13.6079(4) to 13.727(3) Å and from 14.6224(6) to 14.742(6) Å in two different directions; Figure S8).
A possibility for it to undergo an SCO decoupled from crystal packing [15,26] or substrate [73,74] effects, such as in solutions [21,33], was supported by quantum chemical calculations of the species [Fe(L) 2 ] 2+ in solvents found in the three solvatomorphs (acetonitrile and water), as accounted by the CPCM model [46,75] Table 2 and Table S1); the largest difference in the Fe-N bond lengths of 0.085-0.154 Å is observed for the bonds Fe-N Pz in the HS species of the complex. The distortion of the coordination polyhedron towards an edge-bicapped tetrahedron, as quantified by both the 'twist' and 'rotation' angles and the symmetry measures S(O h ) and S(ebcT) (Table S1, Figure 4), is very similar to that from the X-ray diffraction data collected for the solvatomorphs at 120 K. The orientation of the N-aryl substituents, however, features a larger difference in the rotation angle between the different spin states, which is a key contributor to the spin-state behavior in iron(II) complexes with similar N,N -aryl-disubstituted 3-bpp ligands [18,22]. The calculated energy difference between the two spin states of [Fe(L) 2 ] 2+ is 2.4 kcal/mol, favoring the LS state both in acetonitrile and water media. The latter is in an agreement with the LS state of the iron(II) ion in two out of the three solvatomorphs, with lattice-solvent effects behind its HS state in [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) either through [76] geometrical distortions of the iron(II) species or, more likely [13], intermolecular interactions. Such a small difference between the two spin states of [Fe(L) 2 ] 2+ additionally hints on a possibility for the complex [Fe(L) 2 ](BF 4 ) 2 to undergo an SCO but under different conditions, e.g., in a solution.
To observe this hypothetical SCO by the NMR spectroscopy often used [21] for the design of SCO-active compounds [16,21,[77][78][79][80], a mixture of the solvatomorphs [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red), [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red), and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) produced by our attempts to grow single crystals of [Fe(L) 2 ](BF 4 ) 2 was dissolved in acetonitrile-d 3 , which provides a good solubility of the complex and occurs in two of the three solvatomorphs. As the popular NMR-based technique in the SCO research [21], the Evans method [27], requires a studied compound to be isolated and thoroughly purified, the mixture was dried under high vacuum to remove the lattice solvents before the sample preparation. 1 H NMR spectra collected for the resulting acetonitrile-d 3 solution at the room temperature showed a set of six paramagnetically shifted signals indicative of the complex [Fe(L) 2 ](BF 4 ) 2 in the HS state.
To assist with the signal assignment, which is sometimes a challenge for the paramagnetic compounds [81], we used an approach [28] successfully applied to various transition metal complexes [81][82][83], including those with 3-bpp ligands [22,[84][85][86]. At its core is a separation of the chemical shifts for a paramagnetic complex into diamagnetic (δ dia ), contact (δ cs ) and pseudocontact (δ pc ) contributions [45]. The former two are easily obtained from the NMR spectra of the corresponding free ligand [40] and from simple quantum chemical calculations, respectively. To access δ pc , the sum of the three contributions to the chemical shifts δ is fitted to those measured experimentally by varying the value of the magnetic susceptibility tensor anisotropy ∆χ ax . For [Fe(L) 2 ](BF 4 ) 2 , such a fit resulted in a good match ( Figure 5) between the chemical shifts in the 1 H NMR spectrum collected from an acetonitrile-d 3 solution of the three (thoroughly dried) solvatomorphs and those estimated by the above approach for the HS species [Fe(L) 2 ] 2+ optimized at TPSSh/def2-TZVP level of theory. Together with a reasonable value of ∆χ ax expected for the HS iron(II) ion in a (pseudo)coordination environment [45], it confirms the HS state of the complex [Fe(L) 2 ](BF 4 ) 2 at the room temperature and its only slightly distorted molecular geometry [22] (Table S1) suggestive of a potential SCO [13]. orientation of the N-aryl substituents, however, features a larger difference in the rotation angle between the different spin states, which is a key contributor to the spin-state behavior in iron(II) complexes with similar N,N′-aryl-disubstituted 3-bpp ligands [18,22]. The calculated energy difference between the two spin states of [Fe(L)2] 2+ is 2.4 kcal/mol, favoring the LS state both in acetonitrile and water media. The latter is in an agreement with the LS state of the iron(II) ion in two out of the three solvatomorphs, with lattice-solvent effects behind its HS state in [Fe(L)2](BF4)2·2CH3CN (yellow) either through [76] geometrical distortions of the iron(II) species or, more likely [13], intermolecular interactions. Such a small difference between the two spin states of [Fe(L)2] 2+ additionally hints on a possibility for the complex [Fe(L)2](BF4)2 to undergo an SCO but under different conditions, e.g., in a solution.
To observe this hypothetical SCO by the NMR spectroscopy often used [21] for the design of SCO-active compounds [16,21,[77][78][79][80], a mixture of the solvatomorphs [Fe(L)2](BF4)2·2CH3CN (red), [Fe(L)2](BF4)2·0.5H2O (red), and [Fe(L)2](BF4)2·2CH3CN (yellow) produced by our attempts to grow single crystals of [Fe(L)2](BF4)2 was dissolved in acetonitrile-d3, which provides a good solubility of the complex and occurs in two of the three solvatomorphs. As the popular NMR-based technique in the SCO research [21], the Evans method [27], requires a studied compound to be isolated and thoroughly purified, the mixture was dried under high vacuum to remove the lattice solvents before the sample preparation. 1 H NMR spectra collected for the resulting acetonitrile-d3 solution at the room temperature showed a set of six paramagnetically shifted signals indicative of the complex [Fe(L)2](BF4)2 in the HS state.
To assist with the signal assignment, which is sometimes a challenge for the paramagnetic compounds [81], we used an approach [28] successfully applied to various transition metal complexes [81][82][83], including those with 3-bpp ligands [22,[84][85][86]. At its core is a separation of the chemical shifts for a paramagnetic complex into diamagnetic (δdia), contact (δcs) and pseudocontact (δpc) contributions [45]. The former two are easily obtained from the NMR spectra of the corresponding free ligand [40] and from simple quantum chemical calculations, respectively. To access δpc, the sum of the three contributions to the chemical shifts δ is fitted to those measured experimentally by varying the value of the magnetic susceptibility tensor anisotropy Δχax. For [Fe(L)2](BF4)2, such a fit resulted in a good match ( Figure 5) between the chemical shifts in the 1 H NMR spectrum collected from an acetonitrile-d3 solution of the three (thoroughly dried) solvatomorphs and those estimated by the above approach for the HS species [Fe(L)2] 2+ optimized at TPSSh/def2-TZVP level of theory. Together with a reasonable value of Δχax expected for the HS iron(II) ion in a (pseudo)coordination environment [45], it confirms the HS state of the complex [Fe(L)2](BF4)2 at the room temperature and its only slightly distorted molecular geometry [22] (Table  S1) suggestive of a potential SCO [13].  For probing its spin-state behavior upon cooling/heating, the Evans technique [27] is a method of choice [21], as it directly measures the magnetic susceptibility of a solution at different temperatures, which are accessible in a chosen solvent, by comparing chemical shifts of this solvent (or another inert substance, such as TMS) to a pure solvent in the NMR spectra recorded simultaneously. For the complex [Fe(L) 2 ](BF 4 ) 2 in acetonitrile-d 3 , the values χT thus obtained are nearly constant at ≈3.6 cm 3 mol -1 K ( Figure 6) between 250 and 345 K, thereby corroborating the HS state of the iron(II) in this temperature range. Below 250 K, however, a small decrease in χT is observed down to 3.4 cm 3 mol -1 K, which may signal an onset of an SCO to the diamagnetic LS state. As it may also potentially arise from another well-known drawback of the Evans method [27], the limited accuracy [87] (up to 10% error), an alternative NMR-based approach [33] was used to confirm an SCO by simply following the temperature dependence of the paramagnetic chemical shifts [33] obtained from the same solution of [Fe(L) 2 ](BF 4 ) 2 as a side-product of the Evans experiment [27]. Relying only on a correct assignment of signals in the NMR spectra, it was successfully applied in a search for new SCO-active compounds among many transition metal complexes [18,[28][29][30][31]33,41,84,[88][89][90] and even in their mixtures [22,32]. For [Fe(L) 2 ](BF 4 ) 2 , however, the paramagnetic chemical shifts nicely follow a linear dependence on the inverse temperature [22,28,29,89,90] (Figure 6) expected [33,41] for systems in an individual spin state. A very minor deviation from this typical Curie behavior, which may arise from the spin state switching [28], is only hinted at the lowest available temperature in acetonitrile.
Crystals 2020, 10, x FOR PEER REVIEW 13 of 19 For probing its spin-state behavior upon cooling/heating, the Evans technique [27] is a method of choice [21], as it directly measures the magnetic susceptibility of a solution at different temperatures, which are accessible in a chosen solvent, by comparing chemical shifts of this solvent (or another inert substance, such as TMS) to a pure solvent in the NMR spectra recorded simultaneously. For the complex [Fe(L)2](BF4)2 in acetonitrile-d3, the values χT thus obtained are nearly constant at ≈3.6 cm 3 mol -1 K (Figure 6) between 250 and 345 K, thereby corroborating the HS state of the iron(II) in this temperature range. Below 250 K, however, a small decrease in χT is observed down to 3.4 cm 3 mol -1 K, which may signal an onset of an SCO to the diamagnetic LS state. As it may also potentially arise from another well-known drawback of the Evans method [27], the limited accuracy [87] (up to 10% error), an alternative NMR-based approach [33] was used to confirm an SCO by simply following the temperature dependence of the paramagnetic chemical shifts [33] obtained from the same solution of [Fe(L)2](BF4)2 as a side-product of the Evans experiment [27]. Relying only on a correct assignment of signals in the NMR spectra, it was successfully applied in a search for new SCO-active compounds among many transition metal complexes [18,[28][29][30][31]33,41,84,[88][89][90] and even in their mixtures [22,32]. For [Fe(L)2](BF4)2, however, the paramagnetic chemical shifts nicely follow a linear dependence on the inverse temperature [22,28,29,89,90] ( Figure  6) expected [33,41] for systems in an individual spin state. A very minor deviation from this typical Curie behavior, which may arise from the spin state switching [28], is only hinted at the lowest available temperature in acetonitrile. To access lower temperatures, we switched to dichloromethane as another solvent in which the complex [Fe(L)2](BF4)2 is soluble, retains its integrity, and produces very similar NMR spectra in an overlapping temperature range of 235-300 K ( Figure S9). According to the results obtained by the Evans method [27] for the solution of thoroughly dried solvatomorphs of [Fe(L)2](BF4)2 in dichloromethane-d2, there is indeed an SCO observed upon further cooling (Figure 7), as evidenced by a significant decrease in the χT value down to 2.0 cm 3 mol -1 K at 190 K. The latter indicates a gradual population of the diamagnetic LS state, which also follows from a non-Curie behavior of the paramagnetic chemical shifts at the lower temperatures (Figure 7). The corresponding large deviations from the linear dependence [33,41] may only appear from a very fast exchange between the two spin isomers of the SCO-active complex rather than from any other exchange reaction [32] and are, therefore, an unambiguous sign of an SCO occurring in a solution of [Fe(L)2](BF4)2 upon its cooling below 250 K. The thermodynamic parameters of this SCO obtained by fitting with a regular solution model [33] are typical for such complexes (Table 3) [16,18,91]. The resulting midpoint temperature (175 K) is, however, much lower than one found in an acetonitrile-d3 solution of an earlier [18] reported SCO-active iron(II) complex with hydroxyl groups in the same position of N,N′- To access lower temperatures, we switched to dichloromethane as another solvent in which the complex [Fe(L) 2 ](BF 4 ) 2 is soluble, retains its integrity, and produces very similar NMR spectra in an overlapping temperature range of 235-300 K ( Figure S9). According to the results obtained by the Evans method [27] for the solution of thoroughly dried solvatomorphs of [Fe(L) 2 ](BF 4 ) 2 in dichloromethane-d 2 , there is indeed an SCO observed upon further cooling (Figure 7), as evidenced by a significant decrease in the χT value down to 2.0 cm 3 mol -1 K at 190 K. The latter indicates a gradual population of the diamagnetic LS state, which also follows from a non-Curie behavior of the paramagnetic chemical shifts at the lower temperatures (Figure 7). The corresponding large deviations from the linear dependence [33,41] may only appear from a very fast exchange between the two spin isomers of the SCO-active complex rather than from any other exchange reaction [32] and are, therefore, an unambiguous sign of an SCO occurring in a solution of [Fe(L) 2 ](BF 4 ) 2 upon its cooling below 250 K. The thermodynamic parameters of this SCO obtained by fitting with a regular solution model [33] are typical for such complexes (Table 3) [16,18,91]. The resulting midpoint temperature (175 K) is, however, much lower than one found in an acetonitrile-d 3 solution of an earlier [18] reported SCO-active iron(II) complex with hydroxyl groups in the same position of N,N -disubstituted 3-bpp (269 K [18]). With no crystal packing [15,26] or substrate [73,74] effects operating in both cases, the emerged stabilization of the HS state by the ligand L with the t-butyl groups can only be rationalized by the electronic effects of these substituents, as gauged by their Hammett constants [92]. Although an electron-donating t-butyl group in this position of an isomeric ligand, 1-bpp, stabilizes the LS state of the metal ion [16], an opposite trend was sometimes [21,32] observed for the iron(II) complexes of 3-bpp but not confirmed yet, as many of them form H-bonds between their NH groups and counterions/solvents effectively masking the 'molecular' reasons behind the LS/HS stabilization in [Fe(3-bpp)] 2+ [62,[93][94][95][96].
Crystals 2020, 10, x FOR PEER REVIEW 14 of 19 disubstituted 3-bpp (269 K [18]). With no crystal packing [15,26] or substrate [73,74] effects operating in both cases, the emerged stabilization of the HS state by the ligand L with the t-butyl groups can only be rationalized by the electronic effects of these substituents, as gauged by their Hammett constants [92]. Although an electron-donating t-butyl group in this position of an isomeric ligand, 1bpp, stabilizes the LS state of the metal ion [16], an opposite trend was sometimes [21,32] observed for the iron(II) complexes of 3-bpp but not confirmed yet, as many of them form H-bonds between their NH groups and counterions/solvents effectively masking the 'molecular' reasons behind the LS/HS stabilization in [Fe(3-bpp)] 2+ [62,[93][94][95][96].

Conclusions
In our search for new SCO-active compounds by a combined use of X-ray diffraction and NMR spectroscopy [18,22,32,[84][85][86], an iron(II) complex of N,N′-disubstituted 3-bpp ligands was found to adopt different spin states in a series of three different solvatomorphs [Fe(L)2](BF4)2·2CH3CN (red), [Fe(L)2](BF4)2·0.5H2O (red), and [Fe(L)2](BF4)2·2CH3CN (yellow) as concomitant products of its crystallization in acetonitrile. A variable-temperature NMR study of a solution by the traditional Evans method [27] and by a less popular but more powerful [32] analysis of paramagnetic chemical shifts [33] unambiguously revealed a thermally induced SCO experienced by the complex [Fe(L)2](BF4)2 at the lowest temperatures accessible in acetonitrile. The observed stabilization of the HS state by an electron-donating t-butyl group in this position of 3-bpp conflicts with the trend identified for the iron(II) complexes with isomeric 1-bpp [16]. It, however, resonates with the controversial reports on 3-bpp [21,32] mostly resulting from H-bonds between the NH groups of the ligands and counterions/solvent molecules [62,[93][94][95][96]. They can only be reconciled by a systematic study of SCO-active complexes with N,N′-disubstituted 3-bpp [18], which is very challenging with [Fe(L)2](BF4)2 being only the fourth [18] of this kind.

Conclusions
In our search for new SCO-active compounds by a combined use of X-ray diffraction and NMR spectroscopy [18,22,32,[84][85][86], an iron(II) complex of N,N -disubstituted 3-bpp ligands was found to adopt different spin states in a series of three different solvatomorphs [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (red), [Fe(L) 2 ](BF 4 ) 2 ·0.5H 2 O (red), and [Fe(L) 2 ](BF 4 ) 2 ·2CH 3 CN (yellow) as concomitant products of its crystallization in acetonitrile. A variable-temperature NMR study of a solution by the traditional Evans method [27] and by a less popular but more powerful [32] analysis of paramagnetic chemical shifts [33] unambiguously revealed a thermally induced SCO experienced by the complex [Fe(L) 2 ](BF 4 ) 2 at the lowest temperatures accessible in acetonitrile. The observed stabilization of the HS state by an electron-donating t-butyl group in this position of 3-bpp conflicts with the trend identified for the iron(II) complexes with isomeric 1-bpp [16]. It, however, resonates with the controversial reports on 3-bpp [21,32] mostly resulting from H-bonds between the NH groups of the ligands and counterions/solvent molecules [62,[93][94][95][96]. They can only be reconciled by a systematic study of SCO-active complexes with N,N -disubstituted 3-bpp [18], which is very challenging with [Fe(L) 2 ](BF 4 ) 2 being only the fourth [18] of this kind.

Conflicts of Interest:
The authors declare no conflict of interest.